He achieved financial independence around the age of 30 through real estate investing. 

I am new to rental property investing and I do see the age of properties ranging 7- 20. if the condition of the property good, would I be able to process to pursue 20 years old one as well?

Especially being at the age of 23.

I know because of my age people might not take me as serious, so I want to get as much knowledge under my belt as I can.Do you guys have any advice on what I should be studying now?

The only thing that can't be forced is time -- the age of the oldest CC is always part of that formula, so it probably does make sense for your son to get a (small) CC that he uses only for small purchase and always, always, always pays on time..

Again, we don't want to be slum lords, we want our tenants to feel at home and happy.Very early on, we noticed she was testing the waters to see what she could get away with and also, I don't think she likes us holding her accountable to the rules and being around her age or younger.

You would need to look at your age and if doing estate type planning or general investing with the goal at some point to 1031 exchange into something else as a strategy.

List source with owners over 45 years in age with at least 60 percent in equity.

So your age has a lot to do with this question.

Asking price is $130K, but this is high considering age (built in 1991), location, repairs needed, and the other properties around it.